Digital electronic systems rely on precise clocks for sequencing among operating states. Higher-speed systems often require faster and faster clock rates.
Crystal oscillators are often used to generate these clocks. A piezoelectric effect causes a crystal such as quartz to vibrate and resonate at a particular frequency. The quartz crystal naturally oscillates at a particular frequency, its fundamental frequency that can be hundreds of megahertz.
The frequency of oscillation can be adjusted somewhat by adjusting the voltage bias to a varactor on the crystal's terminals. However, frequency adjustment is much less than 1 percent. Such voltage-controlled crystal oscillators (VCXO) are popular for their ease of output-frequency adjustment.
While such crystal oscillators or resonators are useful, manufacturing methods may place an upper limit on the available frequency from a crystal. Very high-speed systems may require clocks that are faster than the fundamental clock rates of commonly-available crystal oscillators. Sometimes the clock output can be multiplied, such as by using a phase-locked loop (PLL) or a clock-doubler circuit. However, even doubling the frequency may not be sufficient for higher-speed systems.
PLL frequency multipliers also can generate 2×, 4× or 8× clock frequencies, but the PLL injects much noise into the oscillating signal due to its voltage-controlled oscillator (VCO). The VCO is a low Q block, so its noise spectrum is very much wider than a crystal, which has a large Q or a much narrower noise spectrum. Hence, non PLL frequency multipliers usually have much better noise and jitter performance than PLL frequency multipliers.
The fundamental frequency of oscillation of the crystal may be limited by various factors, such as the geometry of the crystal. Higher frequencies may require thinner crystals that are much more expensive to manufacture. Thus crystals are currently limited to frequencies of less than 200 MHz for inexpensive crystals, or 500 MHz for expensive crystals.
What is desired is a crystal oscillator circuit that outputs a faster clock than the fundamental frequency of the crystal. A crystal oscillator circuit that multiples the fundamental crystal frequency by four or more is desirable.